Problem: Nauuo is a girl who loves playing chess.

One day she invented a game by herself which needs n chess pieces to play on a m× m chessboard. The rows and columns are numbered from 1 to m. We denote a cell on the intersection of the r-th row and c-th column as (r,c).

The game's goal is to place n chess pieces numbered from 1 to n on the chessboard, the i-th piece lies on (r_i,\,c_i), while the following rule is satisfied: for all pairs of pieces i and j, |r_i-r_j|+|c_i-c_j|≥|i-j|. Here |x| means the absolute value of x.

However, Nauuo discovered that sometimes she couldn't find a solution because the chessboard was too small.

She wants to find the smallest chessboard on which she can put n pieces according to the rules.

She also wonders how to place the pieces on such a chessboard. Can you help her?

Input

The only line contains a single integer n (1≤ n≤ 1000) — the number of chess pieces for the game.

Output

The first line contains a single integer — the minimum value of m, where m is the length of sides of the suitable chessboard.

The i-th of the next n lines contains two integers r_i and c_i (1≤ r_i,c_i≤ m) — the coordinates of the i-th chess piece.

If there are multiple answers, print any.

Examples

Input


2


Output


2
1 1
1 2

Input


4


Output


3
1 1
1 3
3 1
3 3

Note

In the first example, you can't place the two pieces on a 1×1 chessboard without breaking the rule. But you can place two pieces on a 2×2 chessboard like this:

<image>

In the second example, you can't place four pieces on a 2×2 chessboard without breaking the rule. For example, if you place the pieces like this:

<image>

then |r_1-r_3|+|c_1-c_3|=|1-2|+|1-1|=1, |1-3|=2, 1<2; and |r_1-r_4|+|c_1-c_4|=|1-2|+|1-2|=2, |1-4|=3, 2<3. It doesn't satisfy the rule.

However, on a 3×3 chessboard, you can place four pieces like this:

<image>

Solution (in JAVA):

Solution:
	import java.io.BufferedReader;
	import java.io.InputStreamReader;
	import java.util.ArrayList;
public class P22 {


	
		public static void main(String args[])
		 {
			 try{
				 
					BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
					
						
					int n=Integer.parseInt(br.readLine());
					ArrayList<Integer> x=new ArrayList<Integer>();
					ArrayList<Integer> y=new ArrayList<Integer>();
					x.add(1);
					y.add(1);
					for(int i=1;i<n;i++)
					{
						if(i%2!=0)
						{
							int x1=2+(i/2);
							int y1=1+(i/2);
							x.add(x1);
							y.add(y1);
						}
						else
						{
							int x1=1+(i/2);
							int y1=1+(i/2);
							x.add(x1);
							y.add(y1);
						}
					}
					
					int max=-1;
					for(int i=0;i<x.size();i++)
					{
						if(x.get(i)>max)
							max=x.get(i);
					}
					System.out.println(max);
					for(int i=0;i<x.size();i++)
					{
						System.out.println(x.get(i)+" "+y.get(i));
					}
					
					}
					catch(Exception e)
					{
						
					}
		 }
	}